With increasing population, disposal of organic waste has become a major cause for concern for public authorities. Indeed, it is well known that if organic wastes are not disposed of appropriately, simply sending them to landfill can lead to serious pollution of surface water and groundwater, and increase the risk of infectious agents.
The production of wastes increases in parallel with development in the standard of living. The consequences of this development are:                an increase in the costs of waste treatment and disposal in order to protect the environment;        an awareness of the limitation on raw materials and energy resources available to carry out the treatments.        
Policies for sorting waste at source have been advocated, with the aim of removing a maximum amount of usable components from the waste in order to reduce problematic landfill tipping or undesirable incineration.
However, as regards the treatment of organic waste, sorting is difficult as both the sources and the type of waste are multiple: waste water, household waste, agri-foodstuffs industrial waste, green waste originating from the upkeep of open spaces, runoff water, etc.
Generally, organic wastes are collected separately, most frequently via the sewer system, and the problem arises mainly in terms of the volume and heterogeneity of the substrates requiring treatment.
A known method of “valorising” or extracting economic value out of organic wastes is their treatment by methanization, which involves treating the wastes by anaerobic fermentation in order to obtain biogas, i.e. a combustible mixture composed of methane and carbon dioxide.
Anaerobic fermentation is one of the natural processes which contribute to the breakdown of dead organic matter into simple gas and mineral elements. It is the result of a complex microbial activity which takes place essentially in two stages:                hydrolysis, by which the macromolecules are decomposed into more simple products; this is a liquefaction or a gasification with conversion of the molecules to fatty acids, salts or also gases.        conversion of its acids, salts or gases into methane and other gases.        
The methanogenic bacteria, grouped under the Methanobacterium, Methanobacillus, Methanococcus, and Methanosarcina genera, form the principal part of the microorganisms involved in this type of fermentation.
They are characterized by a slow growth and live under strict anaerobiosis. Therefore, the conditions which are indispensable to methane synthesis in suitable fermenters called digesters are as follows:                an anaerobic medium, as only decomposition in the absence of air leads to the formation of methane.        a temperature allowing microbial activity between 10 and 65° C., as the enzymes of the methanogenic bacteria are destroyed beyond this temperature. Most frequently, methanization takes place at a temperature comprised between 20 and 40° C.        
These conditions limit the volumes which can be treated, as in addition to containment, there is the time required for decomposition of the waste, which is approximately from a few days to several weeks, according to the volume and quality of the substrate.
Currently, two types of installation for continuous treatment of waste by methanization can be distinguished. The first type of installation is more particularly suitable for solid wastes and is found in technical landfill sites (TLS).
Technical landfill sites are sites of several hectares where wastes are appropriately stored. They are arranged in a favourable hydrogeological framework where it is possible to excavate cells of 10,000 to 1,000,000 m3 where the organic waste is disposed of and covered with earth. A venting device intended to recover the biogas is installed and the landfill gas is extracted by means of wells descending to 20 or 30 meters deep in the multiple layers of waste.
This type of installation requires substantial engineering investment, which is justified by the need to extract the underground gases from the landfill sites, which are greenhouse gases, presenting risks of explosion or soil contamination. These investments greatly exceed the proceeds of the sale of biogas to industry or even to consumers.
The second type of installation, which is more widespread, generally forms an integral part of large-scale conventional water treatment units (CWT) which treat the wastewater produced by hundreds of millions of inhabitants. Here treatments are carried out continuously. Firstly, pretreatment of the wastewater is carried out, allowing degreasing and the removal of matter in suspension, then the sludges obtained are treated by aerobiosis in order to activate the microorganisms contained therein. The wastes in the form of sludges are then introduced into a sealed digester equipped with a bell-type gas holder which makes it possible to trap the biogas under pressure while the organic matter is decomposed by the methanogenic microorganisms.
For reasons of productivity of this system, decomposition of the organic matter cannot be completed in the digester. The residues output from the digesters are therefore in the form of depleted sludges in which the load of organic matter remains too high for it to be discharged into rivers, spread on the fields, or dumped.
Thus on completion of the methanization treatment, these residues must be specifically treated or eliminated.
It will be shown that both types of installation for continuous treatment by methanization of organic wastes mentioned above, are present in large-scale installations, and for the time being do not constitute autonomous treatment units. Methanization units on a smaller scale do exist, for example for producing biogas from pig slurry, but these do not operate continuously.
Whatever the type of installation, it must be emphasized that the biogas produced by methanization is saturated with water and that it comprises other gases apart from methane, which are potentially dangerous. The biogas must therefore undergo treatment or enrichment steps to allow it to be sold, which currently makes its production uneconomic.
Moreover, an organic waste aerobic fermentation process is known, described in U.S. Pat. No. 5,810,903, using mesophilic or thermophilic microorganisms. This fermentation process operates at high temperatures comprised between 50 and 80 degrees, in the presence of an active oxygenation. The microorganisms which proliferate at this temperature allow a rapid breakdown of the organic matter and therefore an effective mineralization of the organic waste in short periods of the order of 24 to 48 hours.
The mesophilic and thermophilic microorganisms also have the advantage of being less sensitive to the pH variations which can arise during breakdown of the organic matter, which occurs in particular when the wastes have different origins.
These microorganisms generally preexist within the various substrates to be treated. Their growth is encouraged by the fact that the fermentation tanks are maintained at a high temperature, using an external heat source.
When these microorganisms are developed within the substrate formed by the wastes, the temperature inside the tanks is maintained in part by the microorganisms themselves, as most of them have an exothermic metabolism.
Under these high-temperature conditions, most of the active substances that can cause safety inconvenience in the recycling of organic wastes, such as medicines—including antibiotics—, synthetic hormones, pesticides, detergents and toxins, are mostly digested or destroyed. A sterilisation of the wastes can also take place during mineralization if the temperature is allowed to increase further. A final product is then obtained in the form of compost toxicologically and microbiologically safe. By compost, it is meant a solid product with dry matter content generally greater than 50% in weight, preferably greater than 70%, the composition of which is stabilized over a week or more.
This final product can be used in agriculture, for example as fertilizing compost in crop growing.
This thermophilic fermentation process is particularly advantageous in that it allows large volumes of organic waste to be converted rapidly, so that they can be converted into a useful organic and non-polluting product.
However, its implementation has certain drawbacks.
Firstly, it requires a considerable input of external energy, due to the need to bring a large volume of waste up to a temperature of around 50° C. This constraint requires heating installations which are not present in the conventional aerobic treatment systems operating at ambient temperature. In addition to the energy required for their operation, these installations impose an extra cost which is deemed excessive in relation to the productivity gain.
Secondly, activation of the fermentation requires an active supply of air having a sufficient oxygen content, which is generally pumped in at the base of the fermentation tanks. Finally, these are considerable volumes of air which have to come into contact with the microorganisms and which are then dispersed into atmospheric air. This air, which can be classified as contaminated, comprises malodorous effluvia which represent a genuine nuisance for residents living near this type of installation.
Thirdly, the wastes treated at the outlet of the tanks must preferably be dehydrated in order to be able to be transported under correct conditions, which, again, increase the energy consumption of this system.
It would therefore be necessary to improve the existing treatment devices using thermophilic microorganisms, in order in particular to make them more attractive in terms of costs and location.
Ideally, the public authorities would like to have available more compact waste treatment units which could be located underground, even in the basements of buildings and which being more numerous, could be better distributed over the country than large-scale units such as those we know today.